A typical application of the invention relates to the management of a battery of an electric vehicle or hybrid vehicle (vehicle associating an electric motor and an engine).
For this type of application at very high energy consumption, batteries having a large storage capacity as well as low dimensions and weight, such as lithium/ion batteries, are required; these batteries provide a voltage which is only a few volts, although one wishes to use electric motors operating at several tens, or indeed several hundreds of volts; this is why several tens of elementary batteries are placed in series, for example 60 batteries whose voltages can vary from 2.2 to 5 volts depending on their state of charge; the 60 batteries can provide a voltage of 130 to 300 volts.
These batteries are fragile and their lifetime depends on their conditions of use; it is necessary that they operate in a clearly determined range of voltages, neither too high nor too low, and it is necessary that they all operate and age in the same manner, failing which the battery which ages quickest will cause a malfunction, or indeed a destruction, of the whole set. Moreover, to reduce the risks of explosion, it is necessary to prevent these batteries from being overly charged. All this gives rise to the need to tightly control the voltage of each elementary battery of the set in series, and optionally the need to individually discharge an overly charged battery.
The control of the individual voltages must be very precise; typically the voltage must be measured to within a few millivolts. This control is carried out on the basis of battery management electronic circuits, and the function of these circuits is firstly to individually measure the voltage of each battery, and thereafter to discharge any batteries that might exhibit too high a voltage in absolute terms (because of the risk of explosion) or relative terms with respect to the other batteries (to avoid different aging of the various batteries).
However, individual control of numerous batteries in series is difficult, notably because the integrated circuits with which it would be necessary to carry out this control are not adapted for operating at voltages of several tens of volts. Now, it is necessary to be able to control not only the batteries closest to the lower potential of the set in series, but also those which are in the middle and those which are closest to the upper potential, knowing that the interval between the lower potential and the upper potential may attain several hundred volts. Even if a set of 60 batteries is decomposed into 5 modules of 12 batteries, by appending an integrated control circuit to each module, insulated from the other control circuits, the problem of the high voltage levels (60 volts for example) remains critical, since it is difficult to measure a differential voltage of a few volts to within a few millivolts with a common-mode voltage which can vary between 0 and 60 volts. Even excellent common-mode rejection leaves measurement errors of several millivolts. Additionally, the instrumentation amplifiers cannot be made with transistors supporting such voltages, since these transistors are particular components (so-called “drift mos” transistors) which do not lend themselves to the production of circuits for fine measurement. This difficulty of producing measurement circuits also exists for the production of discharge control circuits: notably if one wishes to control the discharge of the battery at constant current, an electronic circuit is required which operates even for common-mode voltages that are mutually very different. Finally, for the same reasons of the presence of high voltages, even if the group of batteries is divided into five modules of 12 batteries, there is still the need for a global management circuit for the five modules and which has to communicate with individual control circuits associated with each module, and therefore associated with mutually very different operating voltages.
A way of solving the problem of the variable common-mode voltages could consist in using divider bridges between the terminals of a battery to be measured and a general earth, to reduce the common-mode voltage to a level of a few volts, for each of the individual batteries, before applying the differential voltage to a multiplexer and an analog-digital converter. The latter circuit elements can then form part of a standard technology low-voltage, and therefore relatively inexpensive, integrated circuit. But the final accuracy becomes very poor since the divider bridges merely introduce additional errors on account of their natural inaccuracy and especially on account of the division ratio which multiplies the measurement errors by the division ratio. Additionally the current consumption of the divider bridges is continuous, which would not be acceptable in an application to an electric vehicle, and moreover this consumption would be variable from one battery to another, which would not be acceptable either. Additional circuits would then be necessary to cut off the power supply to the divider bridges, which would render the whole set more complex.